The ability to selectively sense H2 is critically important for a broad range of applications spanning energy, defense, aviation, and aerospace. One of the most significant needs is for sensors that are capable of leak detection of H2 at levels up to the lower explosive limit (˜4% in ambient air). A large body of work therefore exists focused on the research and development of sensors for H2 leak detection and safety applications with a number of commercial products available on the market. Additional applications of hydrogen sensors requiring operation at elevated temperatures include monitoring of hydrogen in metallurgical processes as well as monitoring the composition of fuel gas streams in power generation technologies such as gas turbines and solid oxide fuel cells. Measurements of H2 levels dissolved in transformer oil can also enable condition-based monitoring to provide early detection of potential failures with large associated economic and environmental impacts. A broad range of sensor devices and technologies have been applied to hydrogen sensing including chemi-resistive, electrochemical, catalytic, work function, acoustic, and optical-based approaches.
Optical-based sensors are particularly well-suited for H2 sensing due to a number of inherent advantages that include elimination of electrical wiring and contacts at the sensing location, which provides benefits in terms of safety and sensor longevity in potentially explosive atmospheres, harsh environments, and at high temperatures. A large number of optical-based hydrogen sensor devices have been developed and demonstrated, many of which are based upon the changes in optical properties of a functional sensor material. Pd and Pd-alloy thin films are the most common materials employed for optical H2 sensing applications due to a well-known dependence of the optical constants of Pd on ambient H2 concentration. See e.g., Vargas et al., “Optical and electrical properties of hydrided palladium thin films studied by an inversion approach from transmittance measurements,” Thin Solid Films 496 (2006); see also Silva et al., “A Review of Palladium-Based Fiber-Optic Sensors for Molecular Hydrogen Detection,” IEEE Sensors Journal, 12(1) (2012); see also Yang et al., “Fiber Optic Hydrogen Sensors: a Review,” Phototonic Sensors 4(4) (2014), among others. These measurable dependences arise from a large solid solubility of hydrogen ions in the Pd lattice resulting in (1) volume expansion, (2) modifications to free carrier concentration, and (3) alterations to electronic band structure. The overall result is a decrease in the magnitude of the real and imaginary parts of the dielectric constant with increasing H2 in the ambient atmosphere. Pd thin films have also been used in conjunction with optical fibers in an evanescent wave absorption spectroscopy based approach allowing for direct monitoring of changes in the real and imaginary parts of the refractive index. See e.g., Luna-Moreno et al., “Optical fiber hydrogen sensor based on core diameter mismatch and annealed Pd—Au thin films,” Sensors and Actuators B 125 (2007); see also Wei et al., “Nano-structured Pd-long period fiber gratings integrated optical sensor for hydrogen detection,” Sensors and Actuators B 134 (2008), among others. In many cases, Pd alloys have shown advantageous sensing properties relative to elemental Pd due to improved film morphology stability and a reduction in (1) hysteresis, (2) response time, and (3) interference due to other chemical species present by suppressing a phase transformation to the PdHx phase and also tailoring the surface chemistry. See e.g., Luna-Moreno et al, “Tailored Pd—Au layer produced by conventional evaporation process for hydrogen sensing,” Optics and Lasers in Engineering 49 (2011), among others.
In more recent work, it has been demonstrated that Pd or Pd-alloy nanoparticles on the surface of silica substrates or unclad optical fibers can also be utilized for optical H2 sensing. For example, AuPd alloy nanoparticles deposited on single-mode fibers mechanically-thinned to 5- or 10-microns in diameter have shown rapid and monotonic responses to H2 at levels up to the lower explosive limit. See Monzon-Hernandez et al., “Optical microfibers decorated with PdAu nanoparticles for fast hydrogen sensing,” Sensors and Actuators B 151 (2010). In this case, the sensing response was attributed to changes in the effective refractive indices of the particles resulting in a H2-concentration dependent light scattering that increased with increasing H2 concentration at an interrogation wavelength of 1550 nm. Pd nanoparticles on the surface of silica glass substrates have also been synthesized through dewetting of continuous films by high temperature annealing and found to respond optically to H2 when measured in a transmission geometry. See Kracker et al., “Optical hydrogen sensing with modified Pd-layers: A kinetic study of roughened layers and dewetted nanoparticle films,” Sensors and Actuators B 197 (2014). In addition to reversible H2 responses that are presumably associated with modifications to the optical constants of Pd due to H atoms dissolved into the Pd lattice, this work also demonstrated an irreversible change in transmission upon the first H2 exposure that was claimed to be due to reduction of the Pd-nanoparticles oxidized during high temperature annealing. Pd nanoparticle-based H2 sensing layers have a number of potential advantages as compared to continuous thin films of Pd such as (1) response time, (2) sensitivity, and (3) stability of the microstructure at high temperatures or during H2 loading and unloading cycles. Promising early results have been obtained for such systems but additional work is required to more fully understand the mechanistic origin of the sensing response and to explore the effects of elevated temperatures and the presence of other common gas species previously reported to impact H2 sensing responses of Pd-based thin films such as O2 and CO.
The concept of applying top filter layers above a functional sensing layer to overcome cross-sensitivity is very appealing in the multicomponent gas mixtures of real applications. See Fleischer et al., “Selective gas detection with high-temperature operated metal oxides using catalytic filters,” Sensors and Actuators B: Chemical, vol. 69, no. 1, pp. 205-210, 2000. Multilayered thin film systems comprised of nanoparticles embedded in a continuous matrix or nanoparticles with a deposited layer have been employed for optimizing the optical response. See Jiang et al., “Multilayer fiber optic sensors for in situ gas monitoring in harsh environments,” Sensors and Actuators B: Chemical, vol. 177, pp. 205-212, 2013. Zeolites are microporous aluminosilicate crystals which have been widely used in the size-selective separation of gas molecules in the range of 0.3 to 0.4 nm, such as CO2/CH4 and CO2/N2 separations. See Carreon et al., “Alumina-supported SAPO-34 membranes for CO2/CH4 separation,” Journal of the American Chemical Society, vol. 130, no. 16, pp. 5412-5413, 2008. However, there are few studies on H2 separations even by the small pore zeolites NaA because the 8-membered ring pores (0.38 nm) are still too large to separate H2 (0.289 nm) from other gas molecules such as CO2 (0.33 nm), N2 (0.364), CO (0.376 nm), CH4 (0.38 nm) based upon their kinetic diameters. See Liu et al., “NaKA sorbents with high CO2-over-N2 selectivity and high capacity to adsorb CO2,” Chemical Communications, vol. 46, no. 25, pp. 4502-4504, 2010. A recent study provided a simple solution of breaking the structure of zeolites NaA into nano-blocks with smaller size openings by dealumination in acid solutions. See Nishiyama et al., “Hydrogen-permeable membranes composed of zeolite nano-blocks,” Journal of Membrane Science, vol. 306, no. 1, pp. 349-354, 2007. The spin-coated membrane demonstrated a high selectivity for H2/CH4, H2/N2 and H2/CO separations.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.